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Today the right hand front hub stripped it's spline without warning.
The hubs appear to be either a grey cast iron or a nodular iron and I am assuming the axle is a heat treated steel and much stronger that the hub. The splines on the axle look ok so I am just going to put on another hub.
Why would the hub suddenly fail and is it about to happen to the other side as well?

This happened to me shortly after doing my first CV joint on my wagon. I attributed it to not lining up the splines exactly right in the hub and forcing the joint in with a knockometer and then tightening the castle nut. In the 2.68 billion CV joints I have replaced on that car since, I've always greased the outer joint and taken extra care to make sure the splines line up. Never stripped another hub since.

But I haven't had the cv joints out in at least 3 years

If you've done what Tim is suggesting, then it lasted an amazingly long time!
I don't know your driving style compaired with Tim's but lets just say his may have encouraged the problem to arise a little earlier

hmm I sure hope I did mine right all those months ago, I didn't have to hit anything with "Subaru Special Tool #47" (hammer) so hopefully I'm alright!

I cleaned the old hub with some kero and found a small crack between the center of the hub and the outside of the hub. it starts at one of the splines. never seen that before. bratgeebah's hubs look better all the time

I did this twice on my old MY wagon, both times LHS for some reason! I can only put it down to the fact that MY hubs are a little too soft.